The present invention relates to a method of modifying steel slag including free lime (F.CaO), free magnesia (F.MgO), and dicalcium silicate (.UPSILON.-2CaO.SiO.sub.2), and having a liability to raise expansion and disintegration.
Steel slag in molten state, if being cooled as it is, is difficult to dispose and offers no usage other than for fertilizer, because it includes free lime, free magnesia, and .UPSILON.-2CaO.SiO.sub.2 and has a nature of expansion and disintegration.
As the method of treating steel slag described above, it has known to modify such steel slag into stable minerals such as 2CaO.Al.sub.2 O.sub.3.SiO.sub.2 or 2CaO.MgO.2SiO.sub.2 by charging it with silicate-containing minerals or red mud as a modifier as described in U.S. Pat. Nos. 4,711,662 and 4,179,279.
In the method described above, however, there has been a problem that a sufficient modifying reaction is difficult to obtain, because a large amount of modifier consisting of silicate-containing minerals and red mud is charged and does not mix sufficiently with steel slag and, moreover, it absorbs the heat and lowers the temperature of the steel slag.
To solve the problem described above, there has known a method of modifying molten steel slag containing mainly dicalcium silicate by adding boron-containing minerals so as to prevent the powdering of slag in cooling as laid open in U.S. Pat. No. 4,655,831 which is characterized by the addition of above-said boron-containing minerals whose crystal water content is adjusted to 4 to 12% by weight.
Though above-said method is effective to the steel slag containing dicalcium silicate as its major component, there still has been a problem that the method of only adding above-said boron-containing minerals is not effective to the slag in reducing process in electric furnace, the slags such as those produced from continuous casting, etc., and containing free lime and free magnesia in addition to above-said dicalcium silicate, and even the steel slag produced from stainless steel making process if it contains free lime and free magnesia in addition to dicalcium silicate, and the slag after treated still has a nature of expansion and disintegration, and the slag containing heavy metal such as Cr may allow the elusion of Cr.sup.6+ ion therefrom.
As the method to solve these problems, it is known as effective to add silicate-containing minerals further and, as the arts relating to this aspect, there are methods of modifying steel slag published in Japanese Patent laid open Nos. 62-162,657, and 62-87,442.
In Japanese Patent laid open No. 62-162,657, there is proposed a method of modifying steel slag characterized by the modification of steel slag through a fusing reaction raised by adding to the steel slag in molten state with boric acid or boron-containing compounds by the amount by which the content of B.sub.2 O.sub.3 in the slag becomes 0.1 to 0.3% by weight, and one or two and more of modifiers among silicate-containing stones and minerals and their weatherings, regulus, coal ash, glass chips, and foundry waste sand by the amount corresponding to 3 to 5% by weight of the slag. And, as the silicate-containing stones and minerals, it is disclosed to use one or two among obsidian, perlite, liparite, expansive shale, etc.
In above-said method, however, there has been a problem that a smooth reaction is not obtained if the temperature of steel slag in molten state is low.
In Japanese Patent laid open No. 62-87,442, there is proposed a method of modifying steel slag characterized by adding to the steel slag in molten state with boric acid or boron-containing compounds, and at least one kind of modifier among silicate-containing stones and minerals and their weatherings, slag, coal ash, glass chips, and at least one kind of exothermic substance among Al, Si, bark, coal, carbonaceous shale, pulp chips, sawdust, corrugated cardboard, and wastes of packing material.
And, in this method, it is also proposed that above-said exothermic substance is spread into a ladle or put on the bottom of ladle in place of laying slag.
In this case, however, there has been a problem that the exothermic substance does not mix sufficiently with the steel slag and, accordingly, sufficient modifying reaction may not be obtained if the temperature of steel slag is low, and if the exothermic substance is charged by above-said method.
Further, in the conventional method described above, because the basicity (CaO/SiO.sub.2) of slag is not considered in the treatment, there has been a problem that the total amount of CaO becomes large to make the fluidity of slag poor due to its too high basicity even if the specified amount of silicate-containing minerals, etc. is charged and, even if the exothermic substance is charged through bubbling, it does not mix well with the slag, and sufficient modifying reaction may not be obtained.